A SUMO‐dependent Pathway for Cytosolic Protein Quality Control

Abstract

Protein quality control (PQC) maintains proteostasis in cells, whereas PQC dysregulation is found in diseases such as cancers, cardiovascular diseases, and neurodegenerative disorders. Over the past few decades, remarkable progress has been made in understanding compartmentalized PQC pathways that operate inside the nucleus and the endoplasmic reticulum (ER). The small ubiquitin‐related modifiers (SUMOs), for example, were recently found to participate in a nuclear PQC pathway that involves selective sumoylation of misfolded proteins by the SUMO E3 ligase PML, and subsequent recognition and ubiquitylation by the SUMO‐targeted ubiquitin E3 ligase RNF4 (Guo et al., Mol Cell, 2014). Roles of SUMOs in cytoplasmic PQC, however, have not previously been reported. Using yeast mutants and human knockout cell lines, our lab has recently identified a conserved SUMO‐dependent pathway for degradation of a cytosolic, misfolded protein model substrate. The substrate consists of GFP fused to an N‐terminal nuclear export signal (NES) and a C‐terminal hydrophobic degron known as CL1 (NES‐GFP‐CL1). Using protein stability assays, we observed a decrease in the turnover rate of NES‐GFP‐CL1 in a yeast SUMO mutant strain and a human U2OS SUMO1 knockout cell line. Further exploration using nuclear and ER‐localized substrates revealed that this pathway is specific for soluble, cytosolic proteins. In addition, analysis of a U2OS SUMO2 knockout cell line revealed that the observed PQC pathway is specific for the SUMO1 paralogue. This suggests a distinct mechanism from the previously characterized nuclear PML‐SUMO‐RNF4 pathway, which targets misfolded proteins modified by SUMO2/3 polymeric chains. Using cellular fractionation studies, we observed an increase in levels of insoluble NES‐GFP‐CL1 specifically in SUMO1 knockout cells compared to wild type cells. We therefore propose a model where SUMO1 modification of misfolded cytosolic proteins promotes their turnover by enhancing solubility and preventing aggregation. Results from our studies reveal a novel cytosolic PQC regulatory network and provide insights into roles for sumoylation in PQC‐associated diseases.